Photochromic dibenzobarrlenes: long-lived triplet biradical intermediates

Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 +/- 2 s (Ea for decoloration is 14.5 +/- 0.7 kcal mol-1 and log A is 8.92 +/- 0.5 s-1) and highly sensitive to molecular oxygen; the Stern-Volmer quenching constant is 6.9 +/- 0.2 x 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular delta-H abstraction, to yield the triplet biradical intermediate (3)2. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The delta-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical (3)2, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical (3)2 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical (3)2 in the photochromism of appropriately substituted dibenzobarrelenes.     

Rearrangement of 1,3-dipolar cycloadducts derived from bis(phenylazo)stilbene: a DFT level mechanistic investigation

The 1,3-dipolar cycloaddition of bis(phenylazo)stilbene with activated ethene and ethyne derivatives and the subsequent rearrangement of the cycloadducts have been studied using model compounds at the B3LYP/6-31G(d) level of density functional theory (DFT). From the structural and electronic features, a five-membered zwitterionic ring system 9 (1,2,3-triazolium-1-imide system) formed from bis(phenylazo)ethylene is confirmed as the active 1,3-dipole species in the reaction. Formation of the 1,3-dipolar cycloadduct from the alkyne derivative is found to be 26.0 kcal/mol exergonic, and it requires an activation free energy of 19.4 kcal/mol. The 1,3-cycloadduct formed in the reaction undergoes a very facile migration of a nitrogen-bearing fragment, passing through a zwitterionic transition state. A small activation free energy of 8.2 kcal/mol is observed for this step of the reaction, and it is 19.6 kcal/mol exergonic. Further activation of the newly formed rearranged product is possible under elevated temperatures, again passing through a zwitterionic transition state and resulting in the formation of 2,5-dihydro-1,2,3-triazine derivatives. Such derivatives have been recently reported by Butler et al. (J. Org. Chem. 2006, 71, 5679). The charge separation in 9 and the zwitterionic transition states are stabilized through the pi-system of the phenyl rings and the carbonyl groups. Similar structural, electronic, and mechanistic features are obtained for the reaction of 9 with the ethylenic dipolarophile acrylonitrile. Molecular electrostatic potential analyses of the 1,3-dipole and the zwitterionic transitions states are found to be very useful for characterizing their electron delocalization features. The solvation effects can enhance the feasibility of these reactions as they stabilize the zwitterionic transition states to a great extent.     

Thermal and Photochemical Transformations of Hetero-1,3,5-hexatrienes into Five-Membered Rings—Possible Pericyclic Reactions

Heterosubstituted 1,3,5-hexatrienes undergo thermal and photochemical cyclizations, most of which can be viewed as pericyclic reactions. Numerous cyclizations of this type have been reported in the literature. This article presents a systematic account of such reactions, classified according to the number and nature of the heteroatoms.     

Electron transfer reactions. Reaction of dibenzobarrelenes with potassium

The results of our studies on potassium-induced transformations of several bridgehead-substituted dibenzobarrelenes are presented. The dibenzobarrelenes 1b,c,e,f having methyl, hydroxymethyl, methoxy and phenyl groups, respectively, at the bridgehead position and the bridgehead-dimethyl derivative 1g give, on treatment with potassium in THF, the corresponding anthracenes 8b,c,e-g, dihydrodibenzobarreneses 6b,c,e-g and benzoic acid (5). The dibenzobarrelenes 1b,g give, in addition to 8b,g, 6b,g and 5, the corresponding mono-debenzoylated products 7b,g, whereas the methoxy derivative 1e gives both anthraquinone (9) and an enol derivaiive 11, besides 8e, 6e and 5. In contrast, the reaction of the hydroxy derivative 1d with potassium in THF gives a mixture of 5, anthracene (8a), the anthrone 10 and the anthrol 12. To assess the role of oxygen in these reactions, if any, the reactions of some representative substrates such as 1b,d,e with potassium in THF, saturated with oxygen and with potassium superoxide have been studied. Cyclic voltammetric studies have been carried out to measuee the reduction potentials for both one electron and two electron processes, leading to the generation of the corresponding radical anions and dianion intermediates. The radical anions of 1b-g have also been generated pulse radiolytically in methanol and their spectra show absorption maxima in the regions 310-390 and 400-450 nm.     

Nafion-TMS- and TMS-Trifluoroacetate-Induced Rearrangements of Cyclopropyl Ketones - A procedure for the regioselective conversion of semibullvalenes to barrelenes

Trimethylsilyl trifluoroacetate (TMSTFA) and the polymer-supported sulfonate Nafion-TMS exhibit selective reaction modes in the opening of cyclopropyl ketones. The yields are generally high. Nafion-TMS rearranges the tricyclooctanones 1a and 1b to the bicyclooctenones 4a and 4b (while TMSTFA gives ring-opened adducts 6a,b ) with high regioselectivity. Aro-semibullvalenes ( 8a, 14, 17a,b ) are efficiently rearranged to arobarrelenes ( 7a, 13, 20a,b ) by both reagents. The latter rearrangements have also been achieved in mixtures of trifluoroacetic acid and tetramethylsilane (TMS), where the acid combines with an unidentified impurity of commercial TMS to form a strong electrophilic agent. The electrophile-assisted rearrangement of the naphtho-semibullvalenes 14a+b resulted in conversion to the naphthobarrelenes 13a+b with the opposite regioselectivity to that observed for the thermal equilibration at 220°.     

Photoisomerisation of dibenzobarrelenes--a facile route to polycyclic synthons

Triplet state mediated di-pi-methane rearrangements of dibenzobarrelenes give a variety of interesting synthons, formed as primary and secondary photoproducts. These synthons could find use for the synthesis of complex synthetic targets. This tutorial review highlights the photoisomerisation of some bridgehead substituted dibenzobarrelenes and the products derived from them. Selected examples of photoisomerisations proceeding through a tri-pi-methane pathway are also included.     

Halogenated Squaraine Dyes as Potential Photochemotherapeutic Agents. Synthesis and Study of Photophysical Properties and Quantum Efficiencies of Singlet Oxygen Generation*

Abstract— We describe the synthesis and photophysical studies, including measurements of quantum yields of triplet excited states and singlet oxygen generation of bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (2) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (3). These dyes exist in solution in the protonated, neutral, single and double depro-tonated forms, depending on pH. The pK_a values of these dyes were found to be relatively lower than those of the parent bis(2,4,6-trihydroxyphenyl)squaraine (1). Only the single deprotonated forms (Sq) of 2 and 3 showed measurable fluorescence. In microheterogeneous media such as in the presence of β-cyclodextrin, cetyltrunethylammonium bromide and polyvinylpyrrolidone), bathochromic shifts in the absorption and emission spectra of Sq were observed with a substantial enhancement in their fluorescence yields. Triplet excited states are the main transient intermediates obtained upon 532 nm laser excitation of the various forms of 2 and 3 in methanol. These triplets have lifetimes in the range from 0.061 to 132 μs. The triplet quantum yields of double deprotonated forms are low (φT = <0.01), whereas the neutral and Sq^?forms of 2 (φr = 0.12 and 0.22) and 3 (φ_T= 0.24 and 0.5), respectively, exhibited significant triplet yields. Quantum yields of singlet oxygen generation by Sq^?forms of 2 and 3 were determined in methanol and were found to be 0.13 and 0.47, respectively, which are in good agreement with the triplet yields obtained in these systems.